Alumina-based sintered material, containing alumina as a predominant component, has excellent withstand voltage characteristic, heat resistance, mechanical strength, or the like, and is an inexpensive material. Therefore, the sintered material is employed for producing ceramic products, such as a spark plug ceramic insulator (may be referred to simply as “insulator” in the specification) and a multi-layer wiring board of IC packages. Such alumina-based sintered material has been produced by sintering a powder mixture containing a sintering aid including SiO2. One example thereof is an “insulating material” disclosed in U.S. Patent Application No. 2011-0077141 (“Patent Document 1”).
However, when a spark plug insulator is produced from an alumina-based sintered material produced through sintering a raw material with a sintering aid, after sintering, the sintering aid (mainly an Si component) remains at the grain boundary among alumina crystal grains as a low-melting-point glass phase. In this case, the low-melting-point glass phase is softened in a spark plug use environment, such as in a high temperature environment of about 700° C., impairing the withstand voltage characteristics of the insulator. The amount of low-melting-point glass phase in an alumina-based sintered material could be reduced by reducing the amount of the sintering aid added to the raw material. However, in this case, the insulator cannot be densified, or, even when the insulator has been apparently densified, a large number of pores remain in the grain boundary defined by alumina crystal grains, also impairing the withstand voltage characteristics of the insulator.
Meanwhile, internal combustion engines to which a spark plug is to be attached are designed for a small-scale and/or a high-output mode, and increase in area of an intake or an exhaust valve in a combustion chamber, employment of a 4-valve configuration, high compression ratio, etc. have been proposed. Thus, the spark plug which is to be attached to such an internal combustion engine designed for a small-scale and/or a high-output mode must maintain excellent withstand voltage characteristics and have an insulator which is prevented from breakdown and which has high mechanical strength under high-temperature conditions, even though the spark plug is thinned and downsized.
There have already been proposed spark plugs or insulators which meet the above demand. For example, such “spark plugs” and “insulators” are disclosed in Japanese Patent No. 4607253, Japanese Patent No. 4613242, International Publication WO 2011/001699, International Publication WO 2011/001656, International Publication WO 2011/036853, Japanese Patent Application Laid-Open (kokai) No. 2008-24583, and Japanese Patent Application Laid-Open (kokai) No. 2010-208901 (“Patent Documents 2 to 8”).
In recent years, small-scale and/or high-output mode internal combustion engines have been realized more and more. Thus, a spark plug which can exhibit more excellent withstand voltage characteristics and high mechanical strength can satisfactorily realize internal combustion engines of an advanced type such as a small-scale engine. In a small-scale and/or high-output mode internal combustion engine, when the tip of a spark plug exposed to a combustion chamber comes into contact with an easily volatile component vaporized through combustion in the combustion chamber, the surface area of the tip is rapidly cooled. Since alumina crystals and the grain boundary phase defining the grain boundary thereof (e.g., a crystal phase) have different thermal expansion coefficients, the crystal phase in the grain boundary phase serves as a breakage-causing point, thereby causing breakage of the tip of the insulator. In this case, the withstand voltage characteristics and mechanical strength of the spark plug may be impaired at high temperature. Thus, a spark plug, in particular, a spark plug to be attached to a small-scale and/or a high-output mode internal combustion engine, is desired to have further improved withstand voltage characteristics and mechanical strength.
An object of the present invention is to provide a spark plug which exhibits, even at high temperature, excellent withstand voltage characteristics and high excellent mechanical strength, which are comparable to those at ambient temperature.